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I have a water tank on a hill and some lawns to water at the bottom of the hill.

The pipe that connects those two is a 2.5" pipe (yes, really) and when I attach a pressure gauge at the bottom of this 2.5" pipe I see a pressure of about 35 PSI, but when I connect the water tank, the head will be another 10 feet higher (it's a 10 foot tall tank) so ... figure 40 PSI.

I think my flow rate in GPM is extremely high. I tried some calculators online but am not sure if I am getting it right but it appears that I have hundreds and hundreds of GPM. And again, the pressure is 35-40 PSI.

It would be really nice to not have to purchase, install, and maintain a pump for this application, and it seems like I would not need one ... but may I sanity check:

If I connect a sprinkler zone at the bottom, feeding in 35-40 PSI at (god knows what huge GPM) and that sprinkler zone is maybe 200 ft. of 3/4" PVC and say 4 impact sprinkler heads that are designed to run at 30 PSI ... good to go ?

I lose 1-2 PSI on the 200 ft. of 3/4 PVC run ... do I lose any PSI on each of the impact heads (they are in a series) ?

Am I missing something here ?

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  • More information - the sprinklers I am using are the rain bird 2045A Maxi-Paw which can operate anywhere from 25-60 PSI, depending on which nozzle plug you use ... I also have some 35ADJ-TNT-B 3/4 brass impact heads that the spec sheet says can run at 30-60 PSI (with 44 - 51 foot radius, depending...)
    – user227963
    Apr 8, 2019 at 7:03
  • More information goes in your question, not comments. Thanks.
    – isherwood
    Jan 3, 2020 at 15:29
  • Try a sprinkler first to see how far it throws. Most sprinklers won’t go very far with this pressure. Definitely go with a rotating sprinkler (as you have). May 23, 2023 at 12:58
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    10 more feet of tank (which assumes the tank is always full, which seems ...unlikely) is only 4.4 PSI more, not 5.
    – Ecnerwal
    May 23, 2023 at 14:07

5 Answers 5

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The size of your lawn, the throw of the heads, the run time, the number of heads per zone and overlap between their spray patterns, the amount of sun and evaporation during watering, and the water pressure at each head all together determine the answer to your question.

You can twiddle all of these factors except the size of the lawn. A pump is one way to control one of these factors. If you do what you can with all the rest and find that you need an excessive number of zones, maybe a pump will be necessary. There are calculator tools for this.

I haven't tried it but there is a swish-looking phone app planner here.

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The 2.5" pipe down the hill is good for gravity feed (minimal pressure loss due to pipe friction.) The GPM if you opened the 2.5" pipe in that location does not matter unless you do that (and would be at 0 PSI.) The GPM will be the GPM of your sprinkler heads (+ any leaks, so don't leak.)

The 200 feet of 3/4" feed to the sprinklers is likely where the problem will be; much more pipe friction (dynamic head) so pressure when flowing will be lower or much lower. How much lower will depend on the flow rate of the chosen sprinklers. Larger pipe will reduce the dynamic head at any given flow. There are plenty of irrigation calculators to help with that on the web. Here's one with links to others. http://irrigation.wsu.edu/Content/Calculators/General/Pressure-Loss-With-Outlets.php

I get 28 PSI loss (so 12 PSI remaining of your 40) for 200 feet of 0.75" pipe (beware of bad results on that calculator if you enter 3/4 instead) with 4 outlets, and 2.5 GPM per head is pretty low for a lot of sprinklers, but you haven't given a number.

To get to "1 PSI loss in the pipe" as you've guessed, your total flow would be 1.65 GPM so 0.41 GPM per sprinkler head, which seems - unlikely.

You can sort what size lateral pipe you need by applying that calculator (if the outlets are equally spaced - if not, you'll have to get more complex with those calculatons) to your actual sprinkler head flow requirements.

Evidently the first sprinklers you mention have a range from 1.5-8.6 GPM.

4 @ 1.5 GPM = 6 GPM in 1.25" pipe gives just under 1 PSI loss with 4 outlets.

4 @ 8.6 GPM = 34.4 GPM requires 2.5 inch pipe to achieve similar dynamic head, which means you'll have to start accounting for additional head loss in your pipe down the hill at that rate. 2 inch pipe would only be 2.3 PSI loss in the lateral. 1.5" would be 9.5 PSI loss in the lateral and might or might not work for you, depending on minimum required pressure and how much additional you lose in the length of 2.5" pipe down the hill.

The minimum rated pressure (which also makes the circle covered smaller) for your brass ones is 30 PSI with 5.5GPM flow per head, so 22 GPM for four which you might just hit with 1.25" pipe at 10.09 PSI loss. 1.5 inch pipe would be safer at 4.15 PSI loss in the lateral. You'd want to examine the length (not vertical drop) of your 2.5" feed for additional losses, though they might be fairly low still (but I don't know how long that pipe is, as you haven't told us.)

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  • Depending on what you have available for power, or are willing to add via wires or solar, Harper's suggestion of not running all heads at once can also reduce the pipe size you require. Whether the reduced cost of pipe covers the increased cost of automation (or hassle of manually operating valves twice or four times as much) is up to your personal cost-benefit calculations.
    – Ecnerwal
    May 23, 2023 at 14:19
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You shouldn't need a pump with that much head. Here's the secret to getting enough flow to sprinklers. You don't need to run them all at the same time.

So if you have 4 sprinklers and only have the flow to run 2 of them happily, then simply have 2 zones. Run one until it's done. Then run the other.

Best to do it at night - you'll lose less water to evaporation.

Another trick you can do is reduce (ratchet back) the run-time of the sprinklers. Lots of people vastly, vastly over-water their lawns. Reduce the runtime by 15% a week until the lawn starts to show distress... then increase it back up one notch.

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    I think "certainly do not" is a bit strong. If you have 10 zones with four heads each, and if they were installed too far apart for a 35 PSI supply, then perhaps a pump is preferable to adding another 10 zones? Perhaps ... I don't think we have enough info. Nor do we want it! Some lawns? This isn't design my agricultural irrigation system dot com
    – jay613
    May 23, 2023 at 13:46
  • @jay613 Edited. You're right, it depends, but "zones are preferable to a pump" is a given parameter by OP, I didn't make it up lol. StackExchange is not a blog or Youtube... it is a Q&A site. That means nothing stated in any answer is a blanket statement; it is always in the context of OP. May 23, 2023 at 18:41
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From the position of the sprinklers you have 70 feet of head. You have shown that you have sufficient pressure.

What you need to consider is that if you have too many sprinklers then the pressure drops and so does the performance.

You will only need a pump if the area to be irrigated is too large and you need too many sprinklers....

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  • From the research I have done, each individual sprinkler head represents a cumulative drop in FLOW (gpm) but I don't see anyone specifying a pressure drop per head. I feel intuitively that if I have four heads in a row (in a series - not on their own stubs) that pressure PSI must drop after each head but I am having trouble verifying that intuition ... FWIW, each zone will only have 4-6 sprinklers so perhaps I have so few that it won't matter, given my HUGE flow rate ?
    – user227963
    Apr 8, 2019 at 16:14
  • BTW, the reason I think this pipe has a very high flow rate is that I looked at the table on hy-techroofdrains.com/water-flow-through-a-pipe and I see that 2.5" pipe @ 40 psi is 421 GPM ... does that still hold true if pressure is 0psi at the top of the pipe and 40 at the bottom ? Am I misjudging this rough calculation somehow ?
    – user227963
    Apr 8, 2019 at 16:24
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    The inlet pressure changes, have a look at this calculator : engineeringtoolbox.com/flow-liquid-water-tank-d_1753.html
    – Solar Mike
    Apr 8, 2019 at 16:47
  • Can you elaborate ? I am not sure which question or random thought of mine above you were referring to ... Thank you.
    – user227963
    Apr 9, 2019 at 5:22
  • Why not try the calculator with your tank and pipe information...
    – Solar Mike
    Apr 9, 2019 at 5:30
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What is the pressure off your house? 2.5 inch pipe??? Why? If you only have 4 heads on that zone. Also how far away is the main line from valve? Every head on average uses 2 gpm per minute wo 4 heads = 8 gpm a little high but average to a zone. There could be many things wrong with your system the way your describing this issue.

  1. Leak in main line or zone.
  2. If your on farm, clean heads and flush zones. Well or sistern systems will build up rust animal (duck) crap, calcification etc.
  3. Valve not properly firing.
  4. Tree roots smothering pipe. Would make sense showing the flow (gpm) but no pressure.
  5. Mainline doesn't run long enough to other individual valve boxes.

If your water flow is coming from 500 feet away and your zone is 100 feet. You'll be waiting forever and just think your valve is stuck. Options

  1. last 20- 50 feet of zone line reduce pipe size to 1.5 inch from 2.5 inch.
  2. Listen for stuck flow by tree roots.
  3. Continue the main from shut off to every box.
  4. Get rid of maxis go 5000s. 2.5 inch pipe you should have pressure to run 30 heads but your main line will hammer cause you house can't produce that much water fast enough.

If you have 2.5 inch zone lines your main is either same or 3 inch. Which is absolutely absurd. Are you running your system off a fire hydrant? lol. I assume from a well so yes you need a pump.

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    There is no Main line, just a tank on the hill. May 23, 2023 at 12:55
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    May 23, 2023 at 15:12

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